Fix xml serialization test [rails#4650 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

activemodel/test/cases/serializeration/xml_serialization_test.rb

@@ -17,14 +17,17 @@ class Contact < ::Contact
   end
 end
 
+class Customer < Struct.new(:name)
+end
+
 class XmlSerializationTest < ActiveModel::TestCase
   def setup
     @contact = Contact.new
     @contact.name = 'aaron stack'
     @contact.age = 25
     @contact.created_at = Time.utc(2006, 8, 1)
     @contact.awesome = false
-    customer = OpenStruct.new
+    customer = Customer.new
     customer.name = "John"
     @contact.preferences = customer
   end
@@ -104,7 +107,7 @@ def setup
   end
 
   test "should serialize yaml" do
-    assert_match %r{<preferences type=\"yaml\">--- !ruby/object:OpenStruct \ntable:\s*:name: John\n</preferences>}, @contact.to_xml
+    assert_match %r{<preferences type=\"yaml\">--- !ruby/struct:Customer \nname: John\n</preferences>}, @contact.to_xml
   end
 
   test "should call proc on object" do